There are two leading processes developed for dehydrogenation (DH) of propane to propylene (for polypropylene) and (iso)butane to isobutene. They are both operated on a large scale. There are no commercial catalytic processes for ethane to ethylene due to the high temperatures required and coke formation.
A continuous process (UOP Oleflex) first commercialized in 1990, uses a Pt/Sn/Al2O3 catalyst in 3 adiabatic (but close to isothermal) radial flow moving bed reactors with feed pre-heat, inter-stage heating and continuous catalyst regeneration (CCR). The process gets close to thermodynamic equilibrium. Fresh feed is mixed with recycled hydrogen (to reduce coking) and unconverted feed at slightly positive pressure.
Another commercialized process (ABB Lummus Catofin), is a cyclic process that uses a Cr2O3/Al2O3 catalyst (activated alumina impregnated with 18-20 wt % chromium) in 3 fixed bed reactors operating under slight vacuum. While one reactor is processing feed, one has its catalyst regenerated in situ with air and the third is purged to give continuous plant throughput. Fresh and recycle feed are preheated to 550-650° C. (isobutane), or 550-750° C. for propane, and fed to the reactor at 0.35-0.7 bar pressure. During reaction, coke deposits on the catalyst and combustion of the coke during regeneration re-heats the catalyst bed.
The Steam Active Reforming process uses a Pt/Sn/Zn/Al2O3 (aluminate spinel) catalyst with steam diluent to maintain a positive pressure in the reactor and reduce partial pressure of hydrocarbons and hydrogen, favoring equilibrium. Steam also reduces coke formation and supplies heat to the reaction. Reactors are multi-tubular fixed beds in a furnace firebox to supply heat and operate isothermally to boost single pass equilibrium yields. Reactor operation is cyclic with 7 hours on line followed by 1 hour regeneration with air.
Typical parameters of these 3 processes are given in Table 1 for isobutane feeds.
TABLE 1Characteristics of Some Prior Dehydrogenation ProcessesProcessOleflexSTARCatofinLicensorUOPKrupp UhdeABB LummusTemp (C.)550480-630540-650Pressure (bar)>12.5-3.50.35-0.75H2/HC feed ratio30-20Steam/HC ratio04(2-10)0LHSV (h-1)440.4-2  Conversion (%)~3540-5565Selectivity (%)91-9388-9290Heat inputInter-stage heatingFurnaceCatalystregerationRegenerationMoving bedCyclicCyclicCycle time2-7 days7-8 hours5-15 minutesCatalystPt/Sn/Al2O3Pt/Sn/Zn/Al2O3Cr2O3/Al2O3Catalyst life1-2 years1-2 years1.5-3 years
Venkataraman et al., in “Millisecond catalytic wall reactor: dehydrogenation of ethane,” Chem. Eng. Sci., 57, 2335-2343 (2002), studied the dehydrogenation of ethane in a stream of steam and ethane run in a 4 mm inner diameter tube without a catalyst. Heat was added from combustion in a 1 mm gap between the outside of the inner tube and the inner surface of an outer tube. The authors concluded that this reactor, as compared with a conventional steam cracker, gave superior performance in terms of residence time and ethylene yield.
Ethane cracks cleanly at high temperatures; however, for propane and higher hydrocarbons, cracking is known to be less selective and prone to coking.
Wolfrath et al., in “Novel Membrane Reactor with Filamentous Catalytic Bed for Propane Dehydrogenation,” Ind. Eng. Chem. Res., 40, 5234-5239 (2001), studied propane dehydrogenation through a reactor filled with catalytic filaments. Flow through the reactor is illustrated as flowing between the catalyst filaments. In this study, a H2-permeable membrane separates 2 adjacent, filament-filled catalytic beds. During operation, propane was dehydrogenated in one bed while catalyst is regenerated in the adjacent bed. Coking caused significant loss of catalytic activity. Without the membrane, an initial conversion of Xeq=0.24 decreased to 0.14 during the first 100 min on stream and 0.12 after 250 minutes. In the membrane reactor, “propane conversion decreased from 0.34 initially to about 0.2” within the first 50 minutes. The faster rate of deterioration in the membrane reactor was due to faster coke formation as a result of the lower H2 concentration.
Besser et al., in “Hydrocarbon Hydrogenation and Dehydrogenation Reactions in Microfabricated Catalytic Reactors,” which appeared on the internet at attila.stevens-tech.edu/˜xouyang/research.htm, reported on hydrogenation and dehydrogenation of cyclohexane through 0.1 mm×0.1 mm microchannels containing a 20 nm thick Pt layer. The authors observed that the dehydrogenation would not proceed unless some H2 was present initially. The authors also reported that higher temperatures favored dehydrogenation and that above 120° C., a transformation in catalyst conditions occurs which leads to a decline in activity. The authors observed a strong effect of residence time on benzene yield, with increased production at higher residence times. Other than the above-mentioned decrease in catalytic activity above 120° C., this paper does not provide data on coking or the stability of the catalytic system; however, since the experiments were conducted with fresh reactors and data was acquired “as rapidly as possible” to minimize time dependent effects, it appears that the reactors degraded quickly.
Jones et al., briefly reported on the dehydrogenation of pure cyclohexane in a microreactor in which the feed pressure was 150 kPa while the exit pressure was 1 Pa, with a residence time of 1.125 seconds. Conversions were either 7-9% or 2-3%.